The present invention relates to a laser lap welding method for parts made of galvanized steel sheet.
Galvanized sheets are used in many portions of vehicle bodies of automobiles in consideration of corrosion resistance. Panels made of galvanized sheets to form vehicle body parts are press-formed into three-dimensional shapes and are welded together at peripheral portions of such three-dimensional shapes, and thereby form a vehicle body structure based on a hollow cross-section which has an advantage in strength. Generally, such vehicle body panels have been integrated mainly by means of spot welding. However, laser welding is now being introduced as an alternative technique that allows processing at a higher speed.
It is known that vaporized zinc may cause welding failures such as blowholes when galvanized sheets are closely overlapped and welded to each other using a laser, as the vaporized zinc may blow fused metal away or may remain in the fused metal as bubbles. To avoid this problem, JP2571976B and JP10-216974A disclose a technique in which protrusions are formed on any one of galvanized sheets to form a gap for discharging the zinc vapor in a state in which the sheets are overlapped on each other.
Since a vehicle body panel of an automobile is formed by press forming, protrusions are formed on such pressed parts by subjecting the parts to an embossing process with punches arranged inside a press die after a press forming process. However, when performing laser welding on parts originally designed to be suitable for spot-welding, such parts are not designed to allow arrangement of the protrusions necessary for laser welding. As a consequence, there is a problem that the protrusions cannot be located due to limited space and presence of inclinations on a joining surface. There is also another problem that welding positions need to be changed because of the protrusions added.
If the welding positions are changed, a strength performance and an impact resistance performance as a vehicle body structure are changed, whereby the structure will need to undergo performance confirmation tests again. Moreover, the parts that are newly designed cannot directly utilize design data which have been accumulated on the premise of spot welding. Such problems have been considerable obstacles to introduction of laser welding.
Moreover, the press forming is usually unidirectional processing, and the embossing process is also carried out in the same direction as the pressing direction. However, pressed parts for automobiles are formed into three-dimensional shapes with numerous inclined surfaces and curved surfaces. Accordingly, if an embossing process (63) in the same direction as a pressing direction P is carried out on any of these surfaces as shown in FIG. 5A, for example, then the process is not parallel to a processed surface 71.
A protrusion 61 thus processed has an actual apex 61a at a position deviated from the center 63a of a punch 63. Therefore, an amount of projection (g′) of the apex 61a is increased in proportion to deviation 61e, and a proper gap cannot be formed in a state where a mating part 81 is lapped over the protrusion 61 as shown in FIG. 5B (a larger gap g′ than a designed gap g is formed). Furthermore, when this protrusion 61 is clamped between clamps 64 and 65, a clamp force is unevenly applied to the mating part 81 and it is difficult to maintain an even gap between joining surfaces as a holding condition becomes unstable. Hence, fine weld quality may not be obtained.